Offshore platform with sacrificial plug means

ABSTRACT

APPARATUS FOR CARRYING OUT OPERATIONS WITH RESPECT TO AN OFFSHORE PLATFORM ADAPTED TO BE COATED OR LAUNCHED OVER A PRESELECTED WELL SITE AND POSITIONED AT SAID SITE. THE PLATFORM INCLUDES HOLLOW BRACING ELEMENTS FOR STABILIZING LEGS ON THE PLATFORM. SACRIFICIAL PLUGS ARE PROVIDED IN OPERATIVE ASSOCIATION WITH THE BRACING ELEMENTS TO PERMIT THE ENTRY OF SEA WATER INTO THE INTERIORS THEREOF AFTER INSTALLATION OF THE PLATFORM TO ELIMINATE COLLAPSE STRESSES.

Sept. 20, 1971 w, KUBASTA 3,605,416

OFFSHORE PLATFORMWITH SACRIFICIAL PLUG MEANS Filed Oct. 20, 1969 FIG. 2

INVENTOR= JAMES W. KUBASTA HIS ATTORNEY United States Patent 3,605,416 OFFSHORE PLATFORM WITH SACRIFICIAL PLUG MEANS James W. Kubasta, Metairie, La., assignor to Shell Oil Company, New York, NY. Filed Oct. 20, 1969, Ser. No. 867,522 Int. Cl. E02b 17/00 U.S. Cl. 61-465 5 Claims ABSTRACT OF THE DISCLOSURE Apparatus for carrying out operations with respect to an offshore platform adapted to be coated or launched over a preselected well site and positioned at said site. The platform includes hollow bracing elements for stabilizing legs on the platform. Sacrificial plugs are provided in operative association with the bracing elements to permit the entry of sea water into the interiors thereof after installation of the platform to eliminate collapse stresses.

This invention relates to apparatus for performing offshore operations concerned with drilling, producing, servicing and maintaining offshore wells, and pertains more particularly to a platform adapted to be launched and positioned at the well site for carrying out these operations.

In an attempt to locate new oil fields, an increasing amount of well drilling has been conducted at offshore locations, such, for example, as off the coasts of California, Louisiana and Texas, and, more recently, off the coast of Alaska and in the North Sea. According to one approach, such wells are drilled and completed from a platform which is installed on the sea bed at the well site. Such platforms are commonly designed to float, whereby they may be launched at the well site, upended and sunk into position so that the desired operations may be carried out. Flooding is generally accomplished by actuating suitable valves operatively associated with the platform legs to allow sea water to enter the legs, thereby decreasing the buoyancy of the platform until it settles on bottom.

Platform braces, however, normally remain buoyant and watertight even after the platform has been sunk into position. Consequently, the size of such prior art braces was dictated by consideration of a combination of collapse stresses caused by water pressure and axial stresses imposed by storm conditions. By eliminating collapse stresses, the size of the brace members can be substantially reduced thus resulting in significant savings in material utilized in the bracing member construction.

SUMMARY OF THE INVENTION It is therefore a primary object of the present invention to provide a simple and inexpensive approach for eliminating collapse stresses in platform brace elements through the equalization of internal and external pressures applied to said members after installation of the platform.

This and other objects have been attained in the present invention by providing a platform suitable for use in carrying out operations with respect to an offshore well which incorporates a plurality of legs structurally stabilized by means of hollow cross-bracing members. Sacrificial plugs are provided in operative association with the cross-braces to permit the entry of sea water into the interiors thereof after installation of the platform on bottom. The plugs are made of a sacrificial metal, such as aluminum, magnesium, etc., which is adapted to react with the sea water and be destroyed thereby within a relatively short period of time. The consequent flooding of the cross-bracing interiors relieves collapse stresses caused by water pressure after platform installation.

DESCRIPTION OF THE DRAWING Other objects, purposes and characteristic features of the present invention will be obvious from the accompanying drawing and from the following description of the invention. On describing the invention in detail, reference will be made to the accompanying drawing in which like reference characters designate corresponding parts throughout several views, and in which:

FIG. 1 is a diagrammatic view in longitudinal projection illustrating apparatus according to the present invention being positioned at an offshore well site and prior to its installation at said site;

FIG. 2 is an enlarged, detailed longitudinal view illustrating details of a platform leg and operatively associated cross-bracing in accordance with the teachings of the present invention; and

FIGS. 3, 4 and 5 are enlarged, cross-sectional views illustrating alternative forms of sacrificial plugs which may be utilized in carrying out the teachings of the present invention.

Referring now to FIG. 1 of the drawing, a platform 11 which has been modified in accordance with the teachings of the present invention is illustrated as being positioned at a well site by vessels 12 and 13-. Platform 11 will incorporate a work deck to be installed later and includes a plurality of legs 14 for supporting the work deck (not shown). Legs 14 are interconnected by means of a network of cross-bracing members such as members 15 and 16. Legs 14 and the cross-bracing members comprising the interconnecting network are of hollow construction and are formed of steel or other suitable material. The legs are closed at their ends and the legs and cross-bracing members are secured together in water-tight fashion as by means of welding so that platform 11 floats in body of water 21 prior to its installation upon sea bed 22 at the preselected well site. To accomplish such installation, suitable valves (not shown) which are operatively associated with legs 14 are actuated to flood the legs, thus permitting the platform to upend and settle into contact with sea bed 22.

The cross-bracing members, however, remain buoyant and Water-tight and although such buoyancy is not sufiicient to keep platform 11 afloat the water-tight condition of the members means that they are subjected to considerable compressive pressures by the body of water 21 upon sinking of the platform. In prior art platform arrangements therefore, the size of the braces has been governed by a combination of collapse stresses caused by water pressure and axial stresses caused by stormy conditions. To eliminate these collapse stresses, small holes are drilled in low stressed areas of the cross-bracing members during fabrication of the platform. Prior to the launching operation, sacrificial plugs are inserted into the holes to maintain the platform buoyancy required prior to the installation. In FIG. 1, a plurality of such plugs are illustrated with representative plugs designated by means of reference numeral 31.

FIG. 2 illustrates plugs 31 in greater detail. With particular reference to this latter figure it may be seen that each plug 31 comprises a cap 35 and an elongated reduced diameter portion 36 extending from said cap. Plug elongated portion 36 is adapted to extend through holes formed in brace members 15 and 16 as shown with the respective dimensions of the holes and the plug elongated portions being such that a water-tight seal is formed between the plugs and the brace members. Each plug cap is then welded to the respective brace member as at 37. Plugs 31 are constructed of a sacrificial metal, i.e., one that is chemically reactive with the sea water in body of water 21 to gradually dissolve or destroy the plug. Suitable metals for this purpose are aluminum and magnesium, for example. The dimensions and physical and chemical characteristics of the plugs are selected such that the plugs will maintain a fluid-tight seal with the respective operatively associated brace members during the launching and installation operations :and be adapted to be destroyed by the chemical reaction with the sea water within a relatively short time after such installation. In this manner the cross-braces are flooded and the internal and external pressures thereon will be equalized to eliminate collapse stresses. By eliminating these stresses, the size of the brace members required may be substantially reduced, resulting in a significant steel sav- 1ngs.

It is, of course, to be understood that the sacrificial plugs employed may assume a variety of shapes :and be secured to the brace members by a number of different expedients. In FIG. 3, for example, a plug 31a is illustrated as having screw threads 41 formed thereon for cooperative engagement with mating screw threads formed on the wall 42 of a brace member. In 'FIG. 4 a plug 31b is illustrated as being in the form of a rivet passing through brace member 42 and forming a liquid-tight seal therewith in an obvious manner. Finally, in FIG. 5 a plug 310 is shown which is of substantially cylindrical configuration. The plug has an outer diameter substantially smaller than the diameter of the bracing member hole with which it is associated. The annulus formed between the plug and wall 42 is filled with a suitable waterproof adhesive 45 which is utilized to bond the plug to the brace member in a fluid-tight manner until disintegration of the plug by the sea water.

I claim as my invention:

1. Apparatus for carrying out operations with respect to ofishore wells and adapted to be installed at an offshore location, said apparatus comprising:

a plurality of hollow leg members adapted to be secured to work deck means and to extend downwardly therefrom, said leg members being further adapted to be positioned on the sea bed in the vicinity of said wells;

a plurality of hollow cross-bracing members interconnecting and extending between said leg members and fixedly positioned with respect to said leg members, each cross-bracing member being sealed at both ends to define a chamber and having port means formed therein; and

disintegratable plug means operatively associated with said cross-bracing members and disposed in said port means to maintain the chambers in a water-free condition prior to installation of said apparatus and disintegratable in saline water to permit the flooding of said chambers through said port means after installation of said apparatus.

2. The apparatus according to claim 1 wherein said plug means operatively associated with said cross-bracing members comprises a plurality of plugs responsive to immersion in said saline water to disintegrate and permit the flooding of said chambers.

3. The apparatus according to claim 2 wherein said plugs are at least partially constructed of a material adapted to chemically react with and be destroyed by said saline water.

4. The apparatus according to claim 3 wherein said material is aluminum.

5. The apparatus according to claim 3 wherein said material is magnesium.

References Cited UNITED STATES PATENTS 2,397,844 4/1946 Dewhurst 114198X 3,097,495 7/1963 Laborde et a1. 6146.5 3,308,046 3/1967 Suleski 114198X DAVID J. WILLIAMOW SKY, Primary Examiner D. H. CORBIN, Assistant Examiner US. Cl. X.R. 6172.3; 114-198 

